I have noticed that I often have expectations for the text because I know what the Greek is "supposed" to say because I recognize the passage.

In this week's Gospel the I puzzled over the use of aorist verbs in John 14: 29...καὶ νῦν εἴρηκα ὑμῖν πρὶν γενέσθαι, ἵνα ὅταν γένηται πιστεύσητε. I understand that the aorist infinitive γενέσθαι is refering to a type of action, a one time event, rather than a necessarily past action. But I was expecting future verbs for γένηται and πιστεύσητε. Are these two in the aorist because they have to agree with γενέσθαι?

As Wallace says, the subjunctive has to do with "potentiality" (ExSyn, 463), or as Porter puts it, "The subjunctive form is used to grammaticalize a projected realm which may at some time exist and may even now exist, but which is held up for examination simply as a projection of the writer or speaker's mind for consideration" (Idioms, 56-7). Incidentally, the subjunctive functions the same in Spanish, in which both purpose clauses and dependent clauses referring to the future require the subjunctive mood. I assume other languages work the same.

Barry Hofstetter wrote:γένηται is subjunctive because of ὅταν, πιστεύσητε because of ἵνα...

Scott Lawson wrote:Thank you Barry. For some reason ίνα doesn't lead me to expect a subjunctive. I'll dig deeper to see what I'm missing.

Barry, I see that it became a part of Hellenistic usage. I think it's the lack of a temporal force that was wrongly lessening my expectation to find ἴνα with a subjunctive. I note that it can also be used with the future indicative so it seems wrong to me to say that a verb is subjunctive because it is with ἴνα. I see that it is the same with ὅταν and more so since it can also be used not only with the future indicative but also with the present, imperfect, aorist and even the pluperfect!

I think Allan's question about the use of the aorist still stands unanswered since a present subjunctive and the future indicative could have been used.

Barry Hofstetter wrote:γένηται is subjunctive because of ὅταν, πιστεύσητε because of ἵνα...

Scott Lawson wrote:Thank you Barry. For some reason ίνα doesn't lead me to expect a subjunctive. I'll dig deeper to see what I'm missing.

Barry, I see that it became a part of Hellenistic usage. I think it's the lack of a temporal force that was wrongly lessening my expectation to find ἴνα with a subjunctive. I note that it can also be used with the future indicative so it seems wrong to me to say that a verb is subjunctive because it is with ἴνα. I see that it is the same with ὅταν and more so since it can also be used not only with the future indicative but also with the present, imperfect, aorist and even the pluperfect!

I think Allan's question about the use of the aorist still stands unanswered since a present subjunctive and the future indicative could have been used.

The use of the future indicative with ἵνα is fairly rare and highly marked. One should really only be wondering about its presence, not its absence. The subjunctive is the ordinary mood with ἵνα. As for the aspect, the aorist subjunctives ἵνα ὅταν γένηται πιστεύσητε help to sequence the two events (by viewing both events as complete with respect to each other, with an ingressive πιστεύσητε), while present subjunctives would have suggested that the events were overlapping.

Barry Hofstetter wrote:γένηται is subjunctive because of ὅταν, πιστεύσητε because of ἵνα...

Scott Lawson wrote:Thank you Barry. For some reason ίνα doesn't lead me to expect a subjunctive. I'll dig deeper to see what I'm missing.

Barry, I see that it became a part of Hellenistic usage. I think it's the lack of a temporal force that was wrongly lessening my expectation to find ἴνα with a subjunctive. I note that it can also be used with the future indicative so it seems wrong to me to say that a verb is subjunctive because it is with ἴνα. I see that it is the same with ὅταν and more so since it can also be used not only with the future indicative but also with the present, imperfect, aorist and even the pluperfect!

I think Allan's question about the use of the aorist still stands unanswered since a present subjunctive and the future indicative could have been used.

It's normal to use the subjunctive in a purpose clause with ἵνα...

ἵνα (Hom.+) conjunction, the use of which increased considerably in H. Gk. as compared w. earlier times because it came to be used periphrastically for the inf. and impv. B-D-F §369; 379; 388–94 al.; Mlt. index; Rob. index.① marker to denote purpose, aim, or goal, in order that, that, final senseⓐ w. subjunctive, not only after a primary tense, but also (in accordance w. Hellenistic usage) after a secondary tense

Arndt, W., Danker, F. W., & Bauer, W. (2000). A Greek-English lexicon of the New Testament and other early Christian literature (3rd ed.) (475). Chicago: University of Chicago Press.

Read the entire article in BDAG -- yes it can occasionally be used with the future indicative, but it is quite rare.